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class User < ActiveRecord::Base
  has_secure_password
  has_many :photos
  has_many :messages
  has_many :comments

  has_many :commented_messages, through: :comments, 
    source: :commentable, source_type: 'Message'

  has_many :commented_photos, through: :comments, 
    source: :commentable, source_type: 'Photo'

end

class Photo < ActiveRecord::Base
  belongs_to :user
  has_many :comments, as: :commentable
end

class Message < ActiveRecord::Base
  belongs_to :user
  has_many :comments, as: :commentable
end

class Comment < ActiveRecord::Base
  belongs_to :user
  belongs_to :commentable, polymorphic: true
end

Try downloading an running this and compare

http://localhost:3000/messages/1/comments

http://localhost:3000/photos/1/comments

Both map to comments#index, which can then examine its uri / parameters to see which commentable type it was called for.